Those are impressive sites.... There was something like that out near Marfa one day when I landed at the airport there.... It actually was that plus a wall of black - I was arranging to get my plane inside and talking w/ a man named John Bird (famous glider geezer) and he laughed and said that actually glider people liked those kinds of clouds.
All I could think of was getting the plane iside and then getting me inside!
that is the morning glory wave phenomenon. i'd hazard a guess that the cloud is marking the rotor so stay out of the cloud. the laminar flow wave updraft on the upwind side of those clouds have enabled a lot of long distance glider and hang glider flights in Australia.